Welcome to the big city! Castelia City has a lot of things to do, and is the location for your third gym battle. But first, let's get some items.
This city is not actually as big as you might think, with just a few roads; the road along the piers is Oceanfront Road, and the vertical streets are (from left to right), Gym Street, Narrow Street, Mode Street and Castelia Street, leading on the ocean side to Liberty Pier, Prime Pier, Unity Pier and Cruise Dock, and at the far east end near the gate is Thumb Pier.
The Items
At Thumb Pier, a scientist will give you an evolutionary stone, either a Leaf Stone, Fire Stone or Water Stone and will tell you that your elemental monkey can evolve using one of these stones. It's recommended you don't choose the Fire Stone, as you can get one later on in the Desert Resort.
The Pokémon Massage Service is on Castelia Street, across from Game Freak, which is available once a day and will make your Pokémon happier.
A man in the Pokémon Centre will give you an item depending on how many people you've traded with; if this is more than 50, you can obtain a rare Master Ball.
On Mode Street there are two main attractions; the ice cream stand and the Battle Company. The ice cream stand can sell you Casteliacones for just 
100, BUT only on Tuesdays and not during the winter. The Battle Company is a good spot for some training:
- Clerk Ingrid
- Herdier (Lv. 19)
760
- Clerk Clemens
- Timburr (Lv. 19)
- 760
- Scientist Randall
- Roggenrola (Lv. 19)
- 912
- Clerk Alberta
- Petilil (Lv. 18)
- Cottonee (Lv. 18)
- 720
- Clerk Warren
- Basculin (Lv. 17)
- Basculin (Lv. 17)
- Basculin (Lv. 17)
- 680
- Clerk Ivan
- Woobat (Lv. 20)
- 800
- Scientist Samantha
- Munna (Lv. 20)
- 960
- Scientist Steve
- Sewaddle (Lv. 20)
- 960
- Clerk Wade
- Pansage (Lv. 19)
- Pansear (Lv. 19)
- Panpour (Lv. 19)
- 1,520
- Janitor Geoff
- Trubbish (Lv. 20)
- Minccino (Lv. 20)
- 4,000
After defeating Geoff, he will give you a valuable item: the Exp. Share! You can give this to a Pokémon to hold and they will gain experience, even if they don't participate in the battle.
When you go to the Gym, Cheren is just leaving, having defeated Burgh. When Burgh also leaves the building, you follow him to Prime Pier. You meet with Bianca, Burgh and Iris (a gym leader for a city later on). Bianca reveals that her Munna was stolen by Team Plasma. A Team Plasma Grunt suddenly appears and Burgh chases after her, so you can follow him to Gym Street.
When you enter the building opposite from the Gym, Ghetsis is standing there, with...